Not applicable.
Not applicable.
Not applicable.
This invention is directed to a method of emulsion polymerization of siloxane oligomers in which the particle size of siloxane polymer formed in the emulsion can be decreased by decreasing the amount or concentration of ionic surfactant, i.e., anionic surfactant or cationic surfactant, used in preparing the emulsion.
In particular, the invention is directed to a method of emulsion polymerization of siloxane oligomers in which the size of the siloxane polymer particles formed in the emulsion is controlled by the aqueous phase concentration of ionic surfactant and electrolytes, and by the reaction temperature. Narrow uniform particle size distribution emulsions and microemulsions are produced. According to the method, decreasing the aqueous phase concentration of ionic surfactant results in decreased size of siloxane polymer particle formed during emulsion polymerization. Contrary to current understanding, the method of the invention enables one skilled in the art to decrease the particle size of siloxane polymer formed in the emulsion by decreasing, rather than increasing, the amount or concentration of ionic surfactant used in preparing the emulsion.
The production of aqueous silicone emulsions is commonly practiced by one of three general methods. One method is the emulsification of previously formed organosiloxane polymers by use of surfactants, and the application of shearing forces by a mechanical means, i.e., mechanical emulsification. A second method is the suspension polymerization of reactive oligomeric organosiloxanes that involves the mechanical emulsification of the organosiloxane oligomers, followed by polymerization of the oligomer in the emulsion particles to higher molecular weight organopolysiloxanes. In this second method, often referred to as suspension polymerization, the organosiloxane oligomers are not capable of diffusion into or through water, because they are too high in molecular weight to have any solubility in water. A characteristic of suspension polymerization is that the emulsion particle size is achieved during the mechanical emulsification step and does not change during the polymerization process.
The third method is known as emulsion polymerization, and it utilizes organosiloxane precursors, typically cyclosiloxanes or alkoxysilanes, which are compositions capable of diffusion into or through water in their original form or when hydrolyzed. In the process of silicone emulsion polymerization, siloxane polymers are formed from the starting siloxane precursors, and new emulsion particles are formed which contain the siloxane polymers formed in the polymerization process. The new particles are characteristically smaller than the starting droplets of organosiloxane precursor.
A key component enabling reactions to occur in silicone emulsion polymerization is a surface active catalyst which has both the properties of a surfactant and a catalyst, described in the literature generally as a surfactant-catalyst. It is sometimes described as surfcat for the sake of brevity. Surfcats may be formed in situ in the emulsion polymerization process by ion exchange of a strong acid or base catalyst and an ionic surfactant that is the salt of a surface active strong acid or base, respectively. They can also be prepared beforehand by ion exchange of a strong acid or base catalyst and an ionic surfactant that is the salt of a surface active strong acid or base, respectively, in an aqueous solution.
However, little is known about factors that may control particle formation, size, and size distribution in silicone emulsion polymerization. For example, the Journal of Polymer Science, Part C, No. 27, Pages 27-34, (1969), in an article entitled Anionic Emulsion Polymerization of Siloxanes, Weyenberg et al describe the process in some detail, using cyclosiloxanes or alkyltrimethoxysilanes with dodecylbenzene sulfonic acid (DBSA) as the surfcat. The authors document that increasing the concentration of DBSA results in smaller size particles of methylsilsesquioxanes when using methyltrimethoxysilane, but the effect when using cyclosiloxanes is not described.
In this regard, however, it is generally believed by those skilled in the art, that increasing surfactant concentration relative to the material being emulsified, by whatever means, generally results in formation of smaller size particles. This view is also held by those skilled in emulsion polymerization in particular. For example, in organic free-radical emulsion polymerization, it has been shown that particle number is directly proportional to surfactant concentration, and reference may be had to standard texts such as Emulsion Polymerization and Emulsion Polymers, John Wiley and Sons, Page 46, (1997), Edited by Peter Lovell and Mohamed El-Aasser. As particle number increases for a given amount of dispersed phase, particle size consequently decreases.
Organic free radical emulsion polymerization and silicone emulsion polymerization are similar in many respects, although one significant difference is that the former process uses a free radical initiator which is gradually consumed during the polymerization, and the latter process uses a surfcat as described above which is not consumed.
In organic free radical emulsion polymerization, there are three process intervals, each having different characteristic mechanisms, and reference may be had, for example, to such standard texts as Emulsion Polymerizationxe2x80x94A Mechanistic Approach, Robert G. Gilbert, Academic Press Limited, Pages 51-55, (1995). As noted in such texts, the three process intervals are (1) a period of particle formation (Interval I), (2) a period of increasing particle size with no formation of new particles (Interval II), and (3) a period of polymer growth with no change in particle size and number (Interval III). Generally, Interval I is brief and most of the polymerization occurs in Intervals II and III. During Interval I, surfactant concentration in the aqueous phase is greater than the critical micelle concentration (CMC) of the surfactant in water. Interval I ends when the surfactant concentration becomes less than the CMC. Interval II ends when the starting reactant droplets are completely consumed.
There is limited knowledge in organic free-radical emulsion polymerization about the factors that control particle formation, size, and size distribution in Interval I. Although the reaction chemistry is different, the three Intervals I-III also occur in silicone emulsion polymerization. Typical silicone emulsion polymerizations pass through the three periods, and the particle formation period of Interval I is generally brief. Control of the size of particles being formed is not well understood, however.
It is believed that in the present invention, a method has been discovered whereby the duration of the particle formation period, Interval I, is extended to be the entire or predominate process during emulsion polymerization of cyclosiloxanes, and that the size of particles formed is controlled by certain operating parameters. Accordingly, in terms of the present invention, it is possible to control the size of silicone polymer particles being formed during silicone emulsion polymerization.
It should be noted that silicone emulsion polymerization methods were first described by Hyde and Wehrly in U.S. Pat. No. 2,891,920 (Jun. 23, 1959), and then by Findlay and Weyenberg in U.S. Pat. No. 3,294,725 (Dec. 27, 1966). Both US Patents specify that the organosiloxane precursor be emulsified to carry out the process, except in the case of alkoxysilanes which become very water soluble upon hydrolysis. Neither patent discloses a method to make polydiorganosiloxane microemulsions.
U.S. Pat. No. 4,999,398 (Mar. 12, 1991) discloses a method of making polydiorganosiloxane microemulsions by sequentially adding at an effective rate a standard emulsion of a cyclosiloxane, surfactant, and water, to a polymerization medium of water and a polymerization catalyst, to form a clear, stable, microemulsion having an average particle size less than 0.15 micron. Comparison examples are present in the 398 patent where the cyclosiloxane is not first emulsified prior to beginning emulsion polymerization. Such examples illustrate the failure to achieve clear microemulsions without the cyclosiloxane emulsification step. Each method requires emulsification of the starting siloxane reactant to successfully produce the desired siloxane polymer emulsion or microemulsion.
U.S. Pat. No. 5,726,270 (Mar. 10, 1998) describes a process of making polysiloxane emulsions having an essentially mono-modal particle size in the size range of about 50 nanometer to about 2 micron, by feeding over a periods up to 24 hours, an aqueous mixture of siloxane precursor and an acid catalyst-surfactant to a pre-heated aqueous reaction medium optionally containing one or more surfactants. According to the ""270 patent, it is not necessary for the mixture to be an emulsion, but that is preferred, since all examples other than one example in the ""270 patent use a pre-emulsion; and that one example uses two separate feeds, a siloxane feed and an aqueous acid catalyst-surfactant solution feed. Many examples in the ""270 patent evidence severe mixing problems due to high emulsion viscosity, and in some examples, it was not possible to complete the addition of the pre-cursor emulsion to the reaction medium. According to the ""270 patent, one particularly significant aspect of its method is control of the amount of acid catalyst present, and this is accomplished by maintaining a generally constant ratio of the siloxane precursor to the acid catalyst-surfactant, while the siloxane precursor is added to the reaction medium.
Although the ""270 patent indicates that mono-modal particle size emulsions are advantageous to permit higher silicone concentration emulsions at workable emulsion viscosities, this is not the case as taught in U.S. Pat. No. 4,824,877 (Apr. 25, 1989). At equal concentrations of oil phase and surfactants, the viscosity of an emulsion of broad particle size distribution is lower than a mono-disperse emulsion. While emulsion viscosity can be an important factor, it is of greater importance that the emulsion be essentially free of unemulsified oil such as the polysiloxane, so that the oil remains dispersed in the emulsion, for the generally accepted reasons of product uniformity, and prevention of problems associated with oiling out on the surfaces of containers and pipes.
European Patent 459,500 (Mar. 5, 1997) discloses a silicone emulsion polymerization method that solves many of the problems discussed above, and the ""500 patent provides a viable avenue to methods of making stable, oil free polysiloxane emulsions by emulsion polymerization. The method of the ""500 patent eliminates the need of a pre-emulsification step, it avoids mixing difficulties due to high emulsion viscosity during polymerization, and it provides a means of controlling the size of polymer particles produced over a large range, i.e., greater than about 10 nanometer, in particular, 25 nanometer to about 1200 nanometer.
The method described in the ""500 patent basically involves the steps of (A) preparing a mixture of (i) one or more cyclic siloxanes, (ii) one or more nonionic surfactants, (iii) one or more ionic surfactants, (iv) water, and (v) a siloxane polymerization catalyst, and in which the cyclic siloxane is not mechanically pre-emulsified before it is added to mixture (A); and (B) thereafter heating and agitating the mixture at a polymerization reaction temperature until essentially all of the cyclic siloxane is reacted, and a stable oil-free emulsion is formed.
By controlling certain operational parameters in the method of the ""500 patent, one is able to produce an emulsion of a specific type, i.e., a standard emulsion, a fine emulsion, or microemulsion, and also produce a desired particle size in the resulting emulsion. These operational parameters include (i) the reaction temperature, (ii) the amount and type of ionic surfactant, (iii) the amount and type of nonionic surfactant, (iv) the amount of water, (v) the amount of catalyst, and (vi) the presence of optional ingredients such as an alcohol.
Thus, it has been found that increasing the reaction temperature of the mixture in the emulsion polymerization facilitates production of larger size emulsion polymer particles. Increasing the amount of the ionic surfactant present during the emulsion polymerization facilitates production of smaller size emulsion polymer particles. Increasing the amount of the nonionic surfactant present during the emulsion polymerization facilitates production of larger size emulsion polymer particles.
The effect of the nonionic surfactant was quite unexpected, and it is believed that the nonionic surfactant functions to make the particles being formed less stable, and consequently they aggregate to a larger size before becoming stabilized by the ionic repulsion provided by the ionic surfactant.
In the method of the ""500 patent, the presence of the nonionic surfactant is required during the emulsion polymerization process, as well as use of the operating parameters (i) to (v). The use of a nonionic surfactant in the ""500 patent has two disadvantages. One disadvantage is that nonionic surfactants are known to retard the reaction rate in silicone emulsion polymerization, and this causes an increase in processing time. The second disadvantage is that nonionic surfactants that are susceptible to decomposition by hydrolysis by strong acids or strong bases generally cannot be used in that method.
However, there still exists a need for improved silicone emulsion polymerization methods to reduce manufacturing costs and production time, to improve control of particle size and emulsion viscosity during emulsion polymerization, and to provide added flexibility in formulation of silicone emulsions for various market applications.
Conventional wisdom dictates that at least one viable avenue for achieving a smaller particle size of the siloxane polymer formed in an emulsion prepared by the emulsion polymerization of siloxane oligomers is to increase the amount or concentration of the ionic surfactant used in preparing the emulsion. Reference may be had, for example, to the ""500 patent wherein the patentee Gee states:
xe2x80x9cIt has also been found when using the method of the instant invention, that increasing the amount of the ionic surfactant decreases the particle size of the polysiloxane. The ionic surfactant present during the polymerization reaction appears to have the greatest effect on the particle size. Additional ionic surfactant added in the latter part of the polymerization reaction, just prior to or after neutralization, does not appear to greatly affect the particle size. Additional ionic surfactant is optionally added in the latter part of the polymerization reaction as a means for minimizing viscosity. It is possible to have equivalent amounts of ionic surfactant present in the final emulsion yet produce different particle sizes. This can be achieved by adding different amounts of ionic surfactant during the polymerization reaction and adding any additional amounts in the latter part of the polymerization reaction or just prior to neutralization. High levels of the ionic surfactant present during the polymerization reaction will often result in incomplete reactions and the failure to produce an oil-free emulsion. Levels of ionic surfactant which are too small may also cause similar effects. Those skilled in the art will be able to readily determine the levels of ionic surfactant needed to produce the desired emulsionxe2x80x9d.
The patentee Gee explains further that:
xe2x80x9cThe type of ionic surfactant used in forming the emulsion can also effect the particle size of the polysiloxane. Ionic surfactants can be classified by their hydrophilicity (HLB) or by the number of carbons in the alkyl group of the surfactant. By choosing an ionic surfactant with a higher degree of hydrophilicity and holding all other operational parameters constant, a larger particle size will result in the emulsion formed. A higher degree of hydrophilicity is often associated with shorter alkyl chains. An ionic surfactant with a lower degree of hydrophilicity will result in an emulsion with a smaller particle size. It is preferred to use ionic surfactants having an alkyl chain containing 8 or more carbon atomsxe2x80x9d.
Quite unexpectedly, however, it has been discovered that, contrary to this conventional wisdom, one can achieve a smaller particle size of siloxane polymer formed in emulsions prepared by emulsion polymerization, by decreasing rather than increasing, the amount or concentration of ionic surfactant used in their preparation.
This invention is directed to a method of preparing emulsions which provides unexpected benefits, but which is subject to some limitations for achieving the benefits.
One limitation is that no high shear mixing such as homogenization is used during the process. Another limitation is that no nonionic surfactant is used in the process, i.e., nonionic surfactant free. The remaining limitation is that the particle size of unreacted siloxane oligomer prior to its polymerization in the process is greater than 10 micron/10,000 nanometer.
The method purposely avoids emulsification of the starting siloxane oligomer to minimize reaction at the surface of siloxane oligomer emulsion particles due to the high interfacial surface area created by emulsification. It is believed such particles constitute a distinct population of particles that are not completely consumed and can result in a broad or multi-modal particle size distribution after emulsion polymerization. It is also believed that siloxane particles form by polymerization and precipitation of siloxane oligomers in the aqueous phase to form stable polymer particles. The size of the siloxane polymer particles formed is controlled by the reaction temperature, the aqueous phase concentration of ionic surfactant and electrolytes, and the structure of the ionic surfactant.
The unexpected benefits of the invention are (i) the precise control of the size of silicone polymer particles in emulsions and microemulsions produced by emulsion polymerization; (ii) the ability to decrease, rather than to increase, the surfactant content while maintaining a given polymer particle size; (iii) increased formulation flexibility by not using a nonionic surfactant during emulsion polymerization; and (iv) the optional ability to utilize hydrolyzable nonionic surfactants by post addition after emulsion polymerization is complete.
These and other features and benefits of the invention will become apparent from a consideration of the detailed description.
Not applicable.
Generally, the method of this invention is an improvement over methods described in the European Patent 459,500 discussed above. In particular, the invention provides a method to produce an essentially oil free silicone emulsion or microemulsion of controlled particle size that requires no pre-emulsion of the cyclic siloxane, and which does not use a nonionic surfactant during the emulsion polymerization. An additional feature is that it also does not require any semi-continuous adding or feeding of a siloxane precursor or catalyst-surfactant, i.e., surfcat, to the reaction medium, as in the methods described in U.S. Pat. No. 5,726,270 also discussed above.
As a consequence, the starting siloxane droplets are not pre-emulsified, and with sufficient mixing to prevent mixture phase separation, are typically greater than about 10 micron/10,000 nanometer in size. Furthermore, a desired particle size in the resulting emulsion can be realized without the presence of a nonionic surfactant. This is unexpected as the ""500 patent clearly teaches that a nonionic surfactant be present.
While it is known that an increase in the amount of ionic surfactant present during emulsion polymerization facilitates production of smaller size emulsion polymer particles, surprisingly it has now been discovered that, as the amount of ionic surfactant relative to the amount of siloxane is held constant or even decreasing, increasing ionic surfactant concentration in the water can result in larger polymer particle size, and conversely that decreasing ionic surfactant concentration in the water can result in smaller polymer particle size.
This apparent departure from conventional wisdom is believed to occur when the ionic surfactant concentration in the water becomes sufficiently high, that its contribution to the total ionic strength of the aqueous phase has a greater effect than its contribution to ionic repulsion between particles. This is based on the fact that increasing ionic strength of the aqueous phase increases the size at which unstable particles become stable, in accordance with DLVO electrical double layer theory developed by Derjaguin-Landau and Verwey-Overbeek.
One way to vary the surfactant concentration in the aqueous phase is by varying the amount of water present during the emulsion polymerization. Depending on the structure of the ionic surfactant and the concentration range of the ionic surfactant in the aqueous phase, this trend can be one of increasing or decreasing particle size. It is believed that each ionic surfactant has a characteristic lower concentration range from greater than zero to a certain concentration (Cmax), where increasing concentration in the water phase during emulsion polymerization results in a smaller polymer particle size due to ionic repulsion effects; and a characteristic upper concentration range at concentrations greater than Cmax, where increasing concentration in the water phase results in a larger polymer particle size, apparently due to greater effect of increasing ionic strength rather than ionic repulsion.
The presence of ionic salts such as sodium chloride during emulsion polymerization can be detrimental to achieving reductions in particle size, as their presence results in the formation of larger size polymer particles, apparently due to increasing ionic strength of the aqueous phase, and a compression of the thickness of the electrical double layer on the particles. Electrolytes or salts whose counter-ions relative to the surface active ion of the surfcat have a higher charge number, have a greater effect on increasing the size of particles being formed, and reference may be had to Table 4 appearing below.
While the ""500 patent teaches that increasing the reaction temperature generally results in the formation of larger size polymer particles, it has now been found that in some instances, progressively higher temperatures may result in smaller size particles, or may even result in a reversal in the trend in the size of the particles produced. Maintaining a constant reaction temperature during the emulsion polymerization generally results in a narrow particle size distribution, and conversely, wide variations in temperature during the reaction result in broad or even multi-modal particle size distribution.
Thus, the present invention is for an emulsion polymerization process for preparing stable oil free emulsions containing particles of siloxane polymer which involves the preparation of a mixture (I) from (a) a cyclosiloxane or mixture of cyclosiloxanes, (b) an ionic surfactant or mixture of ionic surfactants, (c) no nonionic surfactant, (d) water, and (e) a siloxane polymerization catalyst, with the proviso that the cyclosiloxane is not mechanically pre-emulsified before addition to mixture (I), and with the further proviso that prior to, or at the point of, contact with the catalyst, the cyclosiloxane is present in the mixture (I) as droplets having an average diameter greater than 10 micron/10,000 nanometer.
According to the method, mixture (I) is then maintained at the emulsion polymerization reaction temperature necessary to obtain the desired polymer particle size prior to, or at the time, when the catalyst and cyclosiloxane both become present in mixture (I) and come in contact with each other. Thereafter, mixture (I) is heated and agitated at the emulsion polymerization reaction temperature, until essentially all droplets of siloxane oligomer are consumed by the polymerization reaction and an emulsion of siloxane polymer particles is formed.
Although one feature of the invention resides in the omission of a nonionic surfactant during the emulsion polymerization process, they may be post-added after the process is completed, if desired, for other purposes such as to enhance emulsion wetting ability on various substrates. This flexibility allows the use of hydrolyzable nonionic surfactants such as ethoxylated fatty acid esters and glyceryl esters of fatty acids.
Since another feature of the invention resides in not using a cyclosiloxane emulsion, it should be understood that for purposes of this invention, the term cyclosiloxane emulsion is intended to mean an emulsion of cyclosiloxanes formed by subjecting a mixture of one or more surfactants, water, and cyclosiloxane, to high-shear mixing, such as with violent mechanical agitation, ultrasonic vibration, a colloid mill, a rotor/stator mixer, or an homogenizer. Homogenization is understood in its classical sense which consists in the use of a positive displacement pump and a homogenizing valve, in which the pump, usually a plunger or piston, forces a fluid into the homogenizing valve. Examples of high-shear mixing equipment are known, and reference may be had, for example, to a standard text such as the Encyclopedia of Emulsion Technology, Volume I, Marcel Dekker Inc., edited by Paul Becher, Pages 64-67, (1983). The particle size of such cyclosiloxane emulsions is generally sub-micron.
In contrast, and according to the present invention, the suspended cyclosiloxane droplets are at least 10 micron/10,000 nanometer in size. Therefore, the droplets are relatively unstable, rising at a relatively high velocity in the absence of agitation, and undergo coalescence to larger droplets, and usually the formation of a separate layer over the water phase. Additionally, the interfacial area is relatively low. In spite of this low interfacial area, the rate of polymerization as taught within the present invention is surprisingly high. This high polymerization rate is unexpected in light of the teachings of U.S. Pat. No. 3,294,725.
Furthermore, although U.S. Pat. No. 5,726,270 teaches those skilled in the art that it is necessary to maintain a constant ratio of anionic catalyst-surfactant to silicone monomer to synthesize mono-modal aqueous dispersions of polysiloxanes, according to the present invention, this requirement is not necessary. Mono-modal emulsions and microemulsions of polysiloxane can be prepared with a constant or a variable ratio of anionic surfactant to silicone monomer during emulsion polymerization.
Therefore, using no high-shear mixing, no nonionic surfactant, and starting with a particle size of the unreacted siloxane oligomer prior to its polymerization of greater than 10 micron/10,000 nanometer, emulsions according to this invention are prepared by simply mixing together the siloxane oligomer, an anionic surfactant, a catalyst, and water.
The polymerization process involves opening of the ring of the cyclic siloxane oligomer using an acid or a base catalyst in the presence of water. Upon opening of the ring, polysiloxanes with terminal hydroxy groups are formed. These polysiloxanes then react with each other through a condensation reaction to form siloxane polymers. A simplified representation of the process is shown below for octamethylcyclotetrasiloxane in which Me is CH3: (Me2SiO)4+H2O+Catalystxe2x86x92HOMe2SiOMe2SiOMe2SiOSiMe2OHxe2x86x92HOMe2SiOMe2SiOMe2SiOSiMe2OH+HOMe2SiOMe2SiOMe2SiOSiMe2OHxe2x86x92HOMe2SiO(Me2SiO)6SiMe2OH+H2O. Polymers of higher molecular weight are obtained by allowing this process to continue.
Generally, siloxane oligomers suitable for this process are cyclic monomers of the formula 
where each R is a saturated or unsaturated alkyl group of 1-6 carbon atoms, an aryl group of 6-10 carbon atoms, and n is 3-7. R can optionally contain a functional group which is unreactive in the ring opening and polymerization reaction.
Suitable R groups are methyl, ethyl, propyl, phenyl, allyl, vinyl, and xe2x80x94Rxe2x80x2F. Rxe2x80x2 is an alkylene group of 1-6 carbon atoms or an arylene group of 6-10 carbon atoms, and F is a functional group such as amine, diamine, halogen, carboxy, or mercapto. R can also be xe2x80x94Rxe2x80x2Fxe2x80x2R where Rxe2x80x2 and R are described above and Fxe2x80x2 is a non-carbon atom such as oxygen, nitrogen, or sulfur.
Cyclic siloxanes most useful as oligomers in this invention include such oligomers as hexamethylcyclotrisiloxane, octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), tetramethyltetravinylcyclotetrasiloxane, tetramethyltetraphenylcyclotetrasiloxane, and mixtures thereof.
In addition to the cyclic siloxane, mixture (I) can also contain alkoxysilanes represented by Rxe2x80x3Si(ORxe2x80x2xe2x80x3)3 or (Rxe2x80x2xe2x80x3O)4Si where Rxe2x80x3 is an organic group containing 1-12 carbon atoms such as an unsubstituted alkyl group CaH2a+1 or an aryl group. Rxe2x80x2xe2x80x3 in the hydrolyzable group xe2x80x94(ORxe2x80x2xe2x80x3) is an alkyl group containing 1-6 carbon atoms. Most preferred silanes Rxe2x80x2xe2x80x3Si(ORxe2x80x2xe2x80x3)3 are alkoxysilanes with neutral organic groups Rxe2x80x2xe2x80x3.
The tetraalkoxysilanes (Rxe2x80x2xe2x80x3O)4Si are exemplified by tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetrabutoxysilane.
Hydrolyzable water-soluble or partially pre-hydrolyzed alkoxysilanes Rxe2x80x3Si(ORxe2x80x2xe2x80x3)3 with neutral organic groups Rxe2x80x3 are exemplified by methyltrimethoxysilane (MTM), methyltriethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, n-butyltrimethoxysilane, hexyltrimethoxysilane, octyltrimethoxysilane, octyltriethoxysilane, dodecyltrimethoxysilane, dodecyltriethoxysilane, and phenyltrimethoxysilane. Any alcohol generated by the hydrolysis of these types of alkoxysilanes may be removed by distillation or some other suitable means.
Hydrolyzable water-soluble alkoxysilanes Rxe2x80x3Si(ORxe2x80x2xe2x80x3)3 with cationic organofunctional groups Rxe2x80x3 exemplified by amino functional silanes can also be included, if desired.
Any anionic surfactant can be used herein, including but not limited to, sulfonic acids and their salt derivatives. Some representative examples of anionic surfactants are alkali metal sulfosuccinates; sulfonated glyceryl esters of fatty acids such as sulfonated monoglycerides of coconut oil acids; salts of sulfonated monovalent alcohol esters such as sodium oleyl isethionate; amides of amino sulfonic acids such as the sodium salt of oleyl methyl tauride; sulfonated products of fatty acid nitriles such as palmitonitrile sulfonate; sulfonated aromatic hydrocarbons such as sodium alpha-naphthalene monosulfonate; condensation products of naphthalene sulfonic acids with formaldehyde; sodium octahydro anthracene sulfonate; alkali metal alkyl sulfates; ether sulfates having alkyl groups of eight or more carbon atoms such as sodium lauryl ether sulfate; and alkylaryl sulfonates having one or more alkyl groups of eight or more carbon atoms such as hexadecylbenzene sulfonic acid and C20 alkylbenzene sulfonic acid.
Commercial anionic surfactants which can be useful in this invention include dodecylbenzene sulfonic acid sold under the name BIOSOFT S-100 by Stepan Company, Northfield, Ill.; the sodium salt of dodecylbenzene sulfonic acid sold under the name SIPONATE DS-10 by Alcolac Inc., Baltimore, Md.; sodium n-hexadecyl diphenyloxide disulfonate sold under the name DOWFAX 8390 by The Dow Chemical Company, Midland, Mich.; and the sodium salt of a secondary alkane sulfonate sold under the name HOSTAPUR SAS 60 by Clariant Corporation, Charlotte, N.C.
Cationic surfactants useful in the invention include compounds containing quaternary ammonium hydrophilic moieties in the molecule which are positively charged, such as quaternary ammonium salts or bases represented by R3R4R5R6N+Xxe2x88x92 where R3 to R6 are alkyl groups containing 1-30 carbon atoms, or alkyl groups derived from tallow, coconut oil, or soy; and X is hydroxide or halogen, i.e., chlorine or bromine. Dialkyl dimethyl ammonium salts which can be used are represented by R7R8N+(CH3)2Xxe2x88x92 where R7 and R8 are alkyl groups containing 12-30 carbon atoms or alkyl groups derived from tallow, coconut oil, or soy; and X is hydroxide or halogen. Monoalkyl trimethyl ammonium salts which can be used are represented by R9N+(CH3)3Xxe2x88x92 where R9 is an alkyl group containing 12-30 carbon atoms or an alkyl group derived from tallow, coconut oil, or soy; and X is halogen.
Representative quaternary ammonium salts and hydroxides are dodecyltrimethyl ammonium chloride/lauryltrimethyl ammonium chloride (LTAC), cetyltrimethyl ammonium chloride (CTAC), didodecyldimethyl ammonium bromide, dihexadecyldimethyl ammonium chloride, dihexadecyldimethyl ammonium bromide, dioctadecyldimethyl ammonium chloride, dieicosyldimethyl ammonium chloride, didocosyldimethyl ammonium chloride, dicoconutdimethyl ammonium chloride, ditallowdimethyl ammonium chloride, ditallowdimethyl ammonium bromide, and cetyltrimethyl ammonium hydroxide. These and other quaternary ammonium salts are commercially available under names such as ADOGEN, ARQUAD, TOMAH, and VARIQUAT.
Any catalyst capable of polymerizing cyclic siloxanes in the presence of water is useful in the method. Catalysts include siloxane polymerization catalysts capable of cleaving siloxane bonds represented by strong acids such as substituted benzene sulfonic acids, aliphatic sulfonic acids, hydrochloric acid, and sulfuric acid. Some anionic surfactants such as dodecylbenzene sulfonic acid can perform the function of an acid catalyst in addition to performing the function of a surfactant, in which case, a separate catalyst is not required. Acid catalysts such as hydrochloric acid and sulfuric acid that are not also surfactants convert the anionic surfactant to an acid by in situ ion exchange of H+ for the surfactant cation, i.e., typically Na+. The anionic surfactant may also be converted to acid form by ion exchange prior to use in the emulsion polymerization process. Cation exchange resins are useful for this purpose.
Other representative siloxane polymerization catalysts include strong bases such as quaternary ammonium hydroxides, and metal hydroxides such as sodium hydroxide and lithium hydroxide. Some examples of suitable quaternary ammonium hydroxides are octadecyltrimethyl ammonium hydroxide, hexadecyloctadecyl dimethyl ammonium hydroxide, and tallow trimethyl ammonium hydroxide. Base catalysts such as sodium hydroxide that are not also surfactants cause in situ ion exchange with quaternary ammonium salts to form quaternary ammonium hydroxides. Cationic surfactants can also be converted to base catalyst surfactants prior to use in emulsion polymerization by ion exchange, and anionic resins are useful for this purpose.
Most typically, emulsions prepared according to this invention contain a silicone polymer concentration of about 10 to 70 percent by weight of the total emulsion solution, preferably about 25 to 60 percent by weight. While emulsions containing less than about 10 percent silicone polymer content can be made, such emulsions hold little or no economic value. The siloxane oligomer can generally be used in the amount of about 1 to 60 percent by weight of the total emulsion. The ionic surfactant is generally present at about 0.05 to 30 percent by weight of the total emulsion, preferably about 0.1 to 20 percent by weight. The catalyst can be present in the reaction medium at a level generally about 0.01 to 30 percent by weight of the total amount of monomer. Strong acids or strong bases can be used within the lower end of this range, while surfactants also capable of functioning as catalysts will be present at a concentration on the higher end of the range. Water constitutes the balance of the emulsion to 100 percent.
Generally, the method of preparing emulsions according to this invention is carried out by first creating a mixture containing the siloxane oligomer, ionic surfactant, no nonionic surfactant, and water. The mixture can then be further processed at room temperature, or it can be heated with agitation, but without high shearing force (homogenization), at a desired polymerization reaction temperature. The catalyst is added to the mixture to initiate polymerization of the oligomer, or any surfactant capable of functioning as the catalyst is activated. The polymerization is allowed to proceed until the siloxane oligomer is consumed and the siloxane polymer formed has reached the desired polymer viscosity or molecular weight.
The catalyst addition step is not necessary if the ionic surfactant is also in an acid or base form. Alternatively, the same general method is used except the initial mixture contains the catalyst, and the siloxane oligomer is added after the desired reaction temperature is established.
Polymerization reaction temperatures are typically above the freezing point, but below the boiling point of water. Pressures above or below atmospheric pressure allow operation outside of this range. At temperatures below room temperature, the polymerization reaction may proceed more slowly. The preferred temperature range is 1-95xc2x0 C., most preferably 50-80xc2x0 C.
The polymerization reaction can be stopped at the desired level of polymerization of the siloxane polymer by using known methods. Reaction times of less than 24 hours, typically less than 10 hours, are sufficient to achieve the desired polymer viscosity.
The methods for stopping the reaction encompass neutralization of the acid or base catalyst by addition of equal or slightly greater stoichiometric amounts of base or acid, respectively. Either a strong or weak base, or a strong or weak acid, may be used to neutralize the reaction. Care must be taken when using a strong base or a strong acid not to over neutralize, as it is possible to re-catalyze the reaction. It is preferred to neutralize with sufficient quantities of base or acid such that the resulting emulsion has a pH of greater than about 7 when an anionic surfactant is present, and a pH of less than about 7 when a cationic surfactant is present.
Some examples of neutralizing agents which may be employed include bases such as sodium hydroxide, ammonium hydroxide, potassium hydroxide, triethanolamine (TEA), triethylamine, isopropyl amine; and acids such as acetic acid and formic acid.
The addition of a preservative may be desirable since emulsions are susceptible to microbiological contamination. Some representative preservatives include compositions such as formaldehyde; 1,3-dimethylol-5,5-dimethyl hydantoin, i.e., DMDM HYDANTOIN; 5-bromo-5-nitro-1,3-dioxane; methyl or propyl paraben; sorbic acid; imidazolidinyl urea; and KATHON CG, i.e., 5-chloro-2-methyl-4-isothiazolin-3-one.
If desired, a small quantity of an alcohol can be added to mixture (I) before or soon after catalysis to increase the particle size of the emulsion. Alcohols useful include methanol, ethanol and isopropanol. Since alcohols are typically used to break emulsions, it is preferred to keep the concentration of the alcohol at low levels, preferably below about three percent by weight. To have the greatest effect on particle size, it is preferred to have the alcohol present throughout the course of the polymerization reaction.